Extruded plastic pipe or tubing is used for a variety of applications. For example, such plastic pipes are utilized for the transportation of water, more specifically delivery systems for hot and/or cold potable water, radiant floor heating, waste water and fire sprinkler systems, among other uses. Such plastic pipes can also be used as district heating pipes and as process pipes in the food industry, and other applications include the conveyance of liquids other than water, such as gases and slurries. Examples of thermoplastic polymers used for the manufacturing of such plastic pipes include polyolefins such as polyethylene (PE) (e.g., PE-raised temperature, or PE-RT), polypropylene (PP), polybutylenes (PB), and any copolymers thereof; polyolefin copolymers such as poly(ethylene-co-maleic anhydride); poly(vinyl chloride) (PVC); and chlorinated PVC, i.e., CPVC; etc. Such thermoplastic polymers may or may not be crosslinked, depending on the polymer system used and the desired properties of the finished pipe.
As one example of a cross-linked polymer, cross-linked polyethylene (PEX) is commonly used for plastic pipes. There are several varieties of PEX that utilize a number of different crosslinking chemistries and processing technologies. Various PEX grades further contain other additives such as antioxidants and/or stabilizer packages in different concentrations and combinations. Three known varieties of PEX for pipe applications are PEX-a, PEX-b, and PEX-c.
In the PEX-a process (“Engel Method”), the crosslinking is induced by peroxide under the influence of heat and high pressure. The resultant PEX-a composition is crosslinked through carbon-carbon bonds to form the crosslinked polymer network. The PEX-a crosslinking process occurs in the melted stage, as opposed to the primary crosslinking processes for PEX-b and PEX-c. The primary reaction is the formation of free radicals upon decomposition of the peroxide, which has to be present by definition for PEX-a, and subsequently, the free radical abstracts hydrogens from the PE polymer chains. The latter gives new carbon radicals, which next combines with neighboring PE chains to form stable carbon-carbon bonds, i.e., crosslinks. The crosslinking, which is considered to be homogeneous and uniform for PEX-a, gives degrees of crosslinking (typically referred to as CCL) in the range of 70-90% for practical applications. Requirement for CCL is to be above 70% for PEX-a as defined in ASTM International's Standard for Crosslinked Polyethylene (PEX) Tubing, F 867-04 (approved May 1, 2004).
In the PEX-b process, the crosslinking is induced by moisture and heat over extended pre-determined times typically conducted in a “Sauna atmosphere”. The most commonly used methods are referred to as the Sioplas (two-steps), and the Monosil (one step) methods, respectively. In the Sioplas method, a silane, such as for example a vinylsilane is grafted to a HDPE resin prior to pipe extrusion. In the Monosil method, a silane is blended with the HDPE resin during pipe extrusion. In both methods, which are chemically different in the pre-crosslinking steps, the fundamental principle for the actual crosslinking are practically identical, i.e., the crosslinking occurs in a secondary post-extrusion process that is accelerated by a combination of heat and moisture. The latter combination is the active “reagent”, which is involved in the primary hydrolysis and condensation reaction. In principle, the extruded pipe is exposed to hot water and a steam bath. A fundamental difference to PEX-a, is that for PEX-b, the resultant crosslinks are not between carbon-carbon bonds, but instead, oxygen-silicon covalent bonds (siloxane “bridges”) are formed. In comparison with PEX-a, the crosslink density (CCL) are somewhat lower for PEX-b (65-70%), and the crosslinking is also less uniform.
In the PEX-c process, the crosslinking is commonly referred to as a “cold” method. In the PEX-c process, no chemicals are needed in order to facilitate the crosslinking process, but instead high energy electron beam (EB) irradiation is utilized to create the free radicals necessary for the hydrogen abstraction and subsequent crosslinking to take place. The high energy electron beams are non-selective, i.e., chemical bonds are cleaved in an un-controlled fashion. The latter has the consequence of creating side reactions, together with the reaction aimed for, i.e., the crosslinking of HDPE. The crosslinking density for PEX-c is typically in the 70-75% range, and caution has to be taken with irradiation time since a too long exposure may give discolored products and/or brittleness. PEX-c has been successfully used for many years despite the somewhat challenging production conditions.
Presently, PEX tubing has temperature and pressure ratings of 160 psi at 73.4° F. (23° C.), 100 psi at 180° F. (82.2° C.), and 80 psi at 200° F. (93.3° C.). Minimum burst ratings are at 475 psi at 73.4° F. (⅝ inch and larger). Additional performance characteristics and requirements for PEX pipes and tubing are given in the Standard for Crosslinked Polyethylene (PEX) Tubing; F 876-04 (approved May 1, 2004) and ISO 9080.
A variety of plastic pipes may be produced in the form of multi-layer plastic pipes, wherein at least one of the layers comprise the extruded thermoplastic plastic pipe as described above. Multi-layer plastic pipes are well known in the industry and have been used for all applications described herein. Additional layers are currently used to provide various desired properties, for example oxygen barrier properties, UV light protection, scratch resistance and enhanced mechanical performance, long-term stability (known as chlorine resistance in accordance with F876 and ASTM 2023), visual appearance in order to create esthetic values and/or for labeling purposes, etc.
In one example, for an oxygen barrier, such additional layers may be produced from thermoplastic non-crosslinked poly(ethylvinylalcohol). For the same purpose, metallic layers can be used, for example aluminum or stainless steel. The metal layer in such cases will provide oxygen barrier properties but also selected visual appearance. In some instances, metal coatings may be applied using vacuum deposition, from which the final metal coatings will have thicknesses in the nanometer range. The metallic layer may also act as a strengthening layer, and in such cases, the metal layer will be thicker, i.e., in the micrometer range. In addition, colored low density polyethylene resins are commonly used to create colored pipes, typically blue for cold potable water applications, and red for hot water. Furthermore, outer coating layers may be applied in the form of crosslinked polyethylene, for example PEX-b.
In any case, where thermoplastic polymers, such as EvOH, polyethylene, PEX-b pre-polymers, etc., co-extrusion technology is commonly used for this purpose. Co-extrusion is a process whereby a coating layer is applied to a polymeric pipe (e.g., a PEX pipe) by extruding a polymer-based material through a ring shaped die as the polymeric pipe is passed through the die. Because of difficulties in obtaining thin coating layers with the co-extrusion process, the practical lower limit for the coating layer thickness is about 100 μm. Co-extrusion also presents other challenges, for example limited flexibility in operating conditions and in potential raw materials, high energy requirements, costly start-up times and purge requirements, and general difficulties with quality control such as obtaining a consistent coating layer thickness and an inability to effectively level the surface of the pipes. In the case where PEX-b technology is used for the outer layers, a secondary time-consuming and costly operation step is necessary.